ZyXEL Communications P-660HW-DX, User Manual

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Chapter 5 WAN Setup
P-660HW-Dx User’s Guide
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5.1.1.3 PPPoA
PPPoA stands for Point to Point Protocol over ATM Adaptation Layer 5 (AAL5). A PPPoA
connection functions like a dial-up Internet connection. The ZyXEL Device encapsulates the
PPP session based on RFC1483 and sends it through an ATM PVC (Permanent Virtual
Circuit) to the Internet Service Provider’s (ISP) DSLAM (digital access multiplexer). Please
refer to RFC 2364 for more information on PPPoA. Refer to RFC 1661 for more information
on PPP.
5.1.1.4 RFC 1483
RFC 1483 describes two methods for Multiprotocol Encapsulation over ATM Adaptation
Layer 5 (AAL5). The first method allows multiplexing of multiple protocols over a single
ATM virtual circuit (LLC-based multiplexing) and the second method assumes that each
protocol is carried over a separate ATM virtual circuit (VC-based multiplexing). Please refer
to the RFC for more detailed information.
5.1.2 Multiplexing
There are two conventions to identify what protocols the virtual circuit (VC) is carrying. Be
sure to use the multiplexing method required by your ISP.
5.1.2.1 VC-based Multiplexing
In this case, by prior mutual agreement, each protocol is assigned to a specific virtual circuit;
for example, VC1 carries IP, etc. VC-based multiplexing may be dominant in environments
where dynamic creation of large numbers of ATM VCs is fast and economical.
5.1.2.2 LLC-based Multiplexing
In this case one VC carries multiple protocols with protocol identifying information being
contained in each packet header. Despite the extra bandwidth and processing overhead, this
method may be advantageous if it is not practical to have a separate VC for each carried
protocol, for example, if charging heavily depends on the number of simultaneous VCs.
5.1.3 Encapsulation and Multiplexing Scenarios
For Internet access you should use the encapsulation and multiplexing methods used by your
ISP. Consult your telephone company for information on encapsulation and multiplexing
methods for LAN-to-LAN applications, for example between a branch office and corporate
headquarters. There must be prior agreement on encapsulation and multiplexing methods
because they cannot be automatically determined. What method(s) you use also depends on
how many VCs you have and how many different network protocols you need. The extra
overhead that ENET ENCAP encapsulation entails makes it a poor choice in a LAN-to-LAN
application. Here are some examples of more suitable combinations in such an application.
5.1.3.1 Scenario 1: One VC, Multiple Protocols
PPPoA (RFC-2364) encapsulation with VC-based multiplexing is the best combination
because no extra protocol identifying headers are needed. The PPP protocol already contains
this information.